1. Field of the Invention
The present invention relates to catalyst powder, an exhaust gas purifying catalyst, and a method of producing the catalyst powder, and particularly, to an exhaust gas purifying catalyst which purifies exhaust gas discharged from an internal combustion engine.
2. Description of the Related Art
In general, a three-way catalyst is used in order to purify exhaust gas which is discharged from an internal combustion engine such as an automotive engine and contains hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxide (NOx). As an example of the three-way catalyst, one in which noble metal particles of platinum (Pt), palladium (Pd), rhodium (Rh) and the like are carried on a carrier of alumina (Al2O3) or the like as a porous body is given.
Moreover, a three-way catalyst in which catalyst powder formed by carrying ones to be described below on active alumina by an impregnation method is carried on a honeycomb substrate is proposed (refer to Japanese Patent Laid-Open Publication No. S59-230639). The ones carried on the honeycomb substrate are: at least one selected from the group consisting of cerium (Ce), zirconium (Zr), iron (Fe) and nickel (Ni); at least one selected from the group consisting of neodymium (Nd), lanthanum (La) and praseodymium (Pr) according to needs; and further, at least one selected from the group consisting of Pt, Pd and Rh. Furthermore, an exhaust gas purifying catalyst is proposed, which is configured such that at least one or more oxides of cobalt (Co), nickel, iron, chromium (Cr) and manganese (Mn) and at least one of Pt, Rh and Pd form a solid solution by the impregnation method on an interface where the above selected substances contact each other (refer to Japanese Patent Publication No. 3251009).
In recent years, in response to tightening of regulations on the exhaust gas, an amount of catalyst to be used per automobile has been increased. An increase of an amount of noble metal to be used per automobile leads to an increase of cost of the automobile, and accordingly, it is desired to reduce the amount of noble metal to be used for the catalyst.
Moreover, a catalytic reaction using the noble metal is a contact reaction which progresses on a surface of the noble metal, and accordingly, a catalytic activity of the noble metal is substantially proportional to a surface area of the noble metal. Therefore, in order to obtain the maximum catalytic activity from a small amount of the noble metal, it is preferable to produce noble metal particles with a small particle diameter and a large specific surface area, and to evenly disperse the noble metal particles on the porous carrier while maintaining the particle diameter.